1. Field of the Invention
The present invention relates to a system of compressing and decompressing image data and, more particularly, to an image encoding/decoding system suitable for a liquid crystal display dynamic capacitance compensation (LCD DDC) apparatus.
2. Description of the Related Art
FIG. 1 is a block diagram of a conventional LCD DCC apparatus. Referring to FIG. 1, the conventional LCD DCC apparatus includes a memory 110 and a lookup table (LUT) module 120. The LCD DCC apparatus applies a voltage higher than a required pixel voltage to a thin film transistor-liquid crystal display (TFT-LCD) panel in order to improve a response time of the TFT-LCD panel. For example, when a current pixel voltage of an LCD panel is A and the next pixel voltage is B, the LCD DCC apparatus does not applies voltages A and B and sequentially supplies voltages A, C and B to the LCD panel. An operation of calculating a voltage for improving a response time of a TFT-LCD panel in the conventional LCD DCC apparatus illustrated in FIG. 1 will now be explained.
The memory 110 receives a current picture and stores the received current picture. When the memory 110 receives the current picture, a previous picture has been stored in the memory 110. The LUT module 120 calculates a voltage required for a target response time of the TFT-LCD panel with reference to a lookup table. Specifically, the LUT module 120 finds out information on a voltage corresponding to a difference between the luminance of a pixel of a currently input picture and the luminance of a corresponding pixel of a previous picture stored in the memory 110 from the lookup table and calculates the voltage required for the target response time of the TFT-LCD panel using the information and the target response time of the TFT-LCD panel.
As described above, the previous picture must be stored in the memory 110 of the LCD DCC apparatus in order to calculate the voltage required for the target response time of the TFT-LCD panel. However, the quantity of data corresponding to a single picture increases as picture quality increases. Accordingly, attempts to compress pictures input to the LCD DCC apparatus and store the compressed pictures in the memory instead of directly storing the pictures in the memory are being made.
Conventional techniques of compressing images include JPEG (Joint Photographic Expert Group), H.264 (intra coding), JPEG-LS and so on. However, these techniques have following programs when applied to LCD DCC.
Firstly, when image data is compressed with compression ratios 1/2 through 1/3 using the above-described techniques, objective picture quality measured using a peak signal-to-noise ratio is not decreased. However, in the case of JPEG or H.264 based on transform coding, subjective picture quality is deteriorated. In particular, an edge of an image becomes indistinct and picture quality at the edge is decreased. Though JPEG can obtain high compression efficiency, picture quality deterioration can be easily confirmed when the picture quality is compared to the picture quality of an original image. Since the LCD DCC apparatus is a circuit for driving a TFT-LCD channel directly shown to people, subjective picture quality is very important in a compression codec for the LCD DCC.
Secondly, the conventional image compression techniques cannot obtain an accurate compression ratio. The conventional image compression techniques can control a compression ratio to a certain extent. However, an accurate compression ratio is required in order to store data in a memory with a limited capacity such as the memory of the LCD DCC apparatus. Particularly, JPEG-LS requires a number of bits greater than the number of bits of an original image because JPEG-LS uses a variable length code.